In recent years, a discharge lamp which does not use mercury and has low power consumption is requested from the viewpoint of environment. However, when mercury is not used and power consumption is low, there is a fear that the discharge becomes unstable or brightness becomes insufficient.
In this case, if the pressure of an inert gas sealed in the discharge space of a light-emitting part of the discharge lamp is increased, the brightness of the discharge lamp can be improved.
Besides, if the size of the light-emitting part is decreased, the brightness of the discharge lamp can be improved.
However, if the pressure of the inert gas sealed in the discharge space of the light-emitting part is increased, although the required brightness can be secured, a new problem arises that the arcuate discharge becomes liable to be deflected up and down by mechanical vibration.
When the arcuate discharge becomes liable to be deflected up and down by the mechanical vibration, there is a fear that flicker occurs in luminous intensity distribution, or a metal halide sealed in the discharge space is flung up and emission color changes.
Besides, if the size of the light-emitting part is decreased, since the distance between the arcuate discharge deflected downward and the metal halide becomes short, the emission color becomes more liable to change.
Then, the development of a discharge lamp is desired which can secure required brightness even at low power consumption (for example, power consumption at the time of stable lighting is 20 W or more and 30 W or less), and the deflection of discharge can be suppressed.